In many applications, such as high power fiber optic industrial lasers and scaleable high power fiber optic phased array laser systems, it is desirable to transmit optical signals having substantial amounts of power via optical fibers. Unfortunately, stimulated Brillouin scattering oftentimes limits the amount of power that can be transmitted via an optical fiber such that, even as additional input power is provided, the output power remains relatively fixed at the threshold at which stimulated Brillouin scattering commences.
In general, stimulated Brillouin scattering is a phase-matched parametric amplification process involving the coupling of a optical wave, an acoustic wave and a backward propagating Stokes wave. In this regard, variations in the index of refraction of an optical fiber induced by pressure differences created by an acoustic wave traveling along the optical fiber can cause a portion of the optical wave to be backscattered, thereby creating the backward propagating Stokes wave. The backward propagating Stokes wave essentially robs power from the optical wave so as to limit the power of the optical signals that can be transmitted via the optical fiber. With reference to quantum physics, stimulated Brillouin scattering can therefore be described by the transfer of a photon from the optical wave into a new Stokes photon of lower frequency and the creation of a new phonon that adds to the acoustic wave.
With reference to FIG. 1, as the input power of the signal transmitted via an optical fiber is increased up to the threshold for stimulated Brillouin scattering, the power level of the signals output by the optical fiber similarly increases as evidenced by positive slope of curve 10. Upon reaching the threshold for stimulated Brillouin scattering, however, further increases in the power of the signals transmitted via the optical fiber will not translate into increased power levels of the optical signals output by the optical fiber. Instead, the power level of the optical signals output via the optical fiber will remain at the threshold at which stimulated Brillouin scattering commences as evidenced by the horizontal portion of curve 10, while the additional input power will be transferred to the backward propagating Stokes wave as shown by the positive slope of curve 12.
Parametric processes, such as stimulated Brillouin scattering, are enhanced in guided wave structures in general, and optical fibers in particular, because the waves that interact, i.e., the optical waves, the acoustic waves, and the Stokes waves, are maintained in the core over relatively long distances. Moreover, stimulated Brillouin scattering is particularly apparent in optical fibers that exhibit a significant overlap of the fundamental optical and acoustic modes within the core of the optical fiber. In this regard, an overlap integral is defined as the integral of the product of the acoustic wave amplitude and the optical wave amplitude over the lateral cross-sectional area of the optical fiber. As the overlap integral approaches unity, coupling between the optical waves and the acoustic waves is at a maximum, thereby resulting in a high level of stimulated Brillouin scattering. As depicted in FIG. 2, for example, a conventional optical fiber having a core doped with GeO2 is susceptible to the early onset of stimulated Brillouin scattering since the fundamental optical and acoustic modes have a 67% mode overlap in the core for an optical wavelength of 1.55 microns and an acoustic frequency of 11.25 GHz. Thus, the forward propagating optical wave of such an optical fiber will couple energy into the formation of a longitudinal acoustic wave which, in turn, can reflect a portion of the power carried by the optical wave back toward the source.
In order to avoid the limitations imposed by the threshold at which stimulated Brillouin scattering commences, optical systems are typically designed such that the optical fibers are operated below the threshold for the onset of stimulated Brillouin scattering. As will be apparent, this approach effectively limits the performance and scalability of the optical systems and may effectively prevent the optical system from being utilized for applications demanding high energy levels. Alternatively, some optical systems utilize a plurality of optical fibers such that the total power handling capability of the plurality of optical fibers satisfies the power requirements of the particular application while ensuring that the power of the optical signals transmitted via each optical fiber is below the threshold at which stimulated Brillouin scattering commences. While facilitating the delivery of optical signals having increased power levels, optical systems of this type obviously include an increased number of components, thereby leading to increased costs and increased weight and volume requirements. Thus, it would be desirable to provide an improved technique for optically transmitting relatively large amounts of power, such as power levels that exceed the threshold at which stimulated Brillouin scattering would commence in a typical optical fiber, such that lasers and other high energy optical systems can be developed without requiring the use of multiple optical fibers that unnecessarily increase the weight and volume of the optical system.